Identification of a transcriptional regulatory module that reduces leaf temperature in poplar under heat stress

A stable leaf temperature provides plants with a suitable microenvironment for photosynthesis. With global warming, extreme temperatures have become more frequent and severe; therefore, it is increasingly important to understand the fine regulation of leaf temperature under heat stress. In this study, five poplar species (Populus tomentosa, Populus simonii, Populus euphratica, Populus deltoides and Populus trichocarpa) that live in different native environments were used to analyze leaf temperature regulation. Leaf temperatures were more stable in Populus simonii and Populus euphratica (adapted to water-deficient regions) under elevated ambient temperature. Although transpiration contributes strongly to leaf cooling in poplar, the thicker epidermis and mesophyll and lower absorbance of Populus simonii and Populus euphratica leaves also help reduce leaf temperature, since their leaves absorb less radiation. Co-expression network and association analysis of a natural population of P. simonii indicated that PsiMYB60.2, PsiMYB61.2 and PsiMYB61.1 play dominant roles in coordinating leaf temperature, stomatal conductance and transpiration rate in response to heat stress. Individuals with CT-GT-GT genotypes of these three candidate genes have significantly higher water-use efficiency, and balance leaf temperature cooling with photosynthetic efficiency. Therefore, our findings have clarified the genetic basis of leaf cooling among poplar species and laid the foundation for molecular breeding of thermostable, water-conserving poplar varieties.